1. Field of the Invention
This invention relates to electro-optical probes used for oscilloscopes that use electro-optical crystals to measure waveforms of signals based on electro-optical effects, and particularly to electro-optical probes used for electro-optic sampling oscilloscopes.
This application is based on Patent Application No. Hei 10-333309 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In general, the electro-optic sampling oscilloscopes operate as follows:
Electric fields being caused to occur due to measured signals are connected with electro-optical crystals, on which laser beams are incident. Using polarization states of the laser beams in the electro-optical crystals, it is possible to detect the measured signals. Herein, the laser beams are formed in a pulse-like form, so that it is possible to measure waveforms of the signals with a very high resolution with respect to time. The electrooptic sampling oscilloscopes use electro-optical probes, which work based on the known electro-optical phenomenon.
As compared with the conventional sampling oscilloscopes using probes of an electric type, the electro-optic sampling oscilloscopes (abbreviated by xe2x80x9cEOSxe2x80x9d oscilloscopes) draw considerable attention of scientists and engineers because of some advantages, as follows:
(1) It is easy to perform measurement on the waveforms of the signals because the EOS oscilloscopes do not require ground lines when measuring the signals.
(2) A metal pin provided at a tip end of the electro-optical probe is insulated from the circuitry, so it is possible to realize high input impedance. Therefore, it is possible to perform measurement without substantially disturbing states of measuring points.
(3) The EOS oscilloscope uses optical pulses for the measurement. So, it is possible to perform the measurement in a broad band, a frequency range of which is increased up to Giga-Hertz (GHz) order.
Now, a description will be given with respect to an example of the EOS oscilloscope with reference to FIG. 3. Specifically, FIG. 3 shows a probe unit 15 of the EOS oscilloscope, which is equipped with a probe head 1 made of an insulator. A metal pin 1a is installed at a center of the probe head 1. An electro-optical element (i.e., electro-optical crystal) 2 is equipped with a reflector (or reflection mirror) 2a, which is formed at a terminal surface facing with an end of the metal pin 1a and is brought into contact with the metal pin 1a. The probe unit 15 contains collimator lenses 3, 10, half-wavelength (or xc2xd wavelength) plates 4, 7, a quarter-wavelength (or xc2xc wavelength) plate 5, polarizing beam splitters 6, 9, and a Faraday rotator 8, which rotates a polarizing plane of incident light by 45 degrees. In addition, the probe unit 15 contains a laser diode 11, which radiates laser beams in response to a control signal output from a main body of the EOS oscilloscope (not shown), as well as photodiodes 12, 13, which convert incoming laser beams to electric signals. Those electric signals are output to the main body of the EOS oscilloscope. Incidentally, the probe unit 15 contains an optical isolator 14a, which is configured by the half-wavelength plates 4, 7, quarter-wavelength plate 5, beam splatters 6, 9 and Faraday rotator 8.
Next, an optical path of the laser beams radiated from the laser diode 11 will be described with reference to FIG. 3, wherein it is denoted by a reference symbol xe2x80x9cCxe2x80x9d.
The collimator lens 10 converts the laser beams output from the laser diode 11 to parallel beams, which propagate straight through the polarizing beam splitter 9, Faraday rotator 8, half-wavelength plate 7 and polarizing beam splitter 6 sequentially in a forward direction. They also pass through the quarter-wavelength plate 5 and half-wavelength plate 4 sequentially. Thereafter, the parallel beams are converged together by the collimator lens 3 and are then incident on the electro-optical element 2 as its incoming beams. The incoming beams of the electro-optical element 2 are reflected by the reflector 2a, which is formed at the terminal surface of the electro-optical element 2 facing with the metal pin 1a. 
Then, reflected beams are converted to parallel beams by the collimator lens 3. The parallel beams propagate through the half-wavelength plate 4 and quarter-wavelength plate 5 in a backward direction. A part of the parallel beams is reflected by the polarizing beam splitter 6 and is incident on the photodiode 12. In contrast, the parallel beams that transmit through the polarizing beam splitter 6 are reflected by the polarizing beam splitter 9 and are incident on the photodiode 13.
The quarter-wavelength plate 4 is provided to make adjustment such that strength of incoming laser beams of the photodiode 12 coincides with strength of incoming laser beams of the photodiode 13. In addition, the half-wavelength plate 4 is provided to adjust a polarizing plane of an incoming beam of the electro-optical element 2.
Next, a description will be given with respect to a series of measuring operations to perform measurement on signals by using the aforementioned probe of the EOS oscilloscope shown in FIG. 3.
When a human operator brings the metal pin 1a in contact with a measuring point (not shown), an electric voltage is applied to the metal pin 1a to form an electric field. Such an electric field spreads and is connected with the electro-optical element 2. Due to Pockel""s effect, there is caused to occur a phenomenon in which a birefringence index changes. The laser diode 11 radiates laser beams, which are incident on the electro-optical element 2. Due to the aforementioned phenomenon, the incoming laser beams that propagate in the electro-optical element 2 change in polarization states. Then, the laser beams whose polarization states are changed are reflected by the reflector 2a and are incident on the photodiodes 12, 13 respectively. The photodiodes 12, 13 convert the incoming beams thereof to electric signals.
Accompanied with changes of the voltage applied to the metal pin 1a at the measuring point, changes occur with respect to the polarization states of the beams in the electro-optical element 2. Those changes bring differences between outputs of the photodiodes 12, 13. By detecting such output differences, it is possible to measure an electric signal being applied to the metal pin 1a. 
Incidentally, the electric signals produced by the photodiodes 12, 13 of the EOS probe are input to the EOS oscilloscope, in which they are processed. Instead of using the EOS oscilloscope, it is possible to use some conventional measurement devices such as the real-time oscilloscope. Herein, the measurement device is connected to the photodiodes 12, 13 by way of a dedicated controller so as to perform measurement on signals. That is, the EOS probe can be widely used for the measurement devices to enable broad-band measurement on the signals with ease.
The aforementioned EOS oscilloscope is designed to separate the incoming beams of the electro-optical element 2, which are brought by the optical isolator 14a, from the reflected beams which are reflected by the reflector 2a. Such a design causes a problem in which a number of optical parts constructing the optical isolator 14a is increased.
Due to an increased number of optical parts, xe2x80x9cunnecessaryxe2x80x9d reflected beams are produced by some optical parts. This causes another problem in which an amount of noise component is increased while a S/N ratio in signal processing is reduced. In addition, there is a still another problem in which the incoming beams of the two photodiodes 12, 13 need to be adjusted in intensities by rotation of the optical parts.
It is an object of the invention to provide an electro-optical probe used for an electro-optic sampling oscilloscope, which is equipped with a reduced number of optical parts and which is improved in S/N ratio.
Basically, an electro-optical probe of this invention is provided for an electro-optic sampling oscilloscope, which is designed as follows:
Electric fields caused by measured signals are connected with an electro-optical crystal, on which optical pulses produced based on timing signals are incident and in which the optical pulses are changed in polarization states. Thus, the electro-optic sampling oscilloscope is capable of measuring waveforms of the measured signals based on changes of the polarization states.
Particularly, this invention provides an improvement in an optical system of the electro-optical probe.
Namely, the electro-optical probe is mainly constructed by a probe head and a probe unit. The probe head contains a metal pin being brought into contact with a measuring point to detect an electric field caused by the measured signal and an electro-optical element having a reflector at its terminal surface facing with an end of the metal pin. The probe unit contains a reduced number of optical parts, which are arranged such that an optical axis of incoming beams of the electro-optical element differs from a optical axis of outgoing beams of the electro-optical element. That is, laser beams radiated from a laser diode propagate along a first optical path and are subjected to convergence by a converging lens to produce converged beams, which are incident on the electro-optical element as its incoming beams. The incoming beams are subjected to reflection by the reflector to produce reflected beams, which are output from the electro-optical element as its outgoing beams along a second optical path. Herein, the first and second optical paths are selected not to be in parallel with each other.
In the electro-optical element, the beams are changed in polarization states in response to the electric field. Then, the reflected beams output from the electro-optical element are converted to parallel beams by a collimator lens and are then input to a polarization detector. Or, they are converged by a converging lens and are then input to the polarization detector. The polarization detector performs separation on the input beams thereof to produce separated components of beams, optical axes of which differ from each other. Herein, a first component of beams substantially corresponding to the input beams is incident on a first photodiode, while a second component of beams corresponding to a part of the input beams is incident on a second photodiode. Thus, the first and second photodiodes respectively output electric signals.
Changes of the polarization states of the beams in the electro-optical element are reflected by differences between the electric signals output from the photodiodes. Thus, it is possible to measure the waveform of the measured signal based on the differences between the electric signals.
Because of the aforementioned arrangement of the optical parts in the probe unit and because of adoption of the polarization detector whose configuration is simple as compared with the conventional optical isolator, it is possible to reduce a total number of the optical parts, while it is possible to improve a S/N ratio in signal processing with respect to measurement of the waveforms of the signals.